1. Field of the Invention
The invention relates in general to a method of channel estimation, and more particularly to a method of channel estimation of an OFDM system.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) has the advantage of anti-multi-path interference and is currently adopted as the specification of digital video broadcasting-terrestrial (DVB-T) transmission.
The OFDM system spread the data to several sub-channels to be transmitted by multi-carrier modulation. The sub-carrier frequency of each sub-channel is different and orthogonal to each other such that each sub-channel can apply a lower transmission rate. Since the sub-carrier frequency of each sub-channel is different, the influence that each sub-channel receives during transmission also differs. The influence that each sub-channel receives is estimated at the reception end. That is, the channel response of the sub-channel is estimated, whereby the received signal is compensated to obtain the correct signal.
There are several methods for estimating channel response such as pilot-based channel estimation for instance. Referring to FIG. 1, a pilot pattern of an OFDM system is shown. Each circle denotes the data transmitted by a sub-channel at a time point, the horizontal axis is sub-channel C, and the vertical axis is time t. At each time point, a group of synchronized signals S including a plurality of signals modulated to the sub-channels are received. The black circle denotes the response signal. The contents of the response signal and the position of the response signal on the frequency-time grid are known to both the transmission end and the reception end. Therefore, the reception end can obtains the channel response of the sub-channel transmitting the response signal by comparing the received response signal with the known response signal.
Other channel responses of the sub-channels transmitting data signals can be obtained by the linear interpolation of the channel response of the known sub-channels. Examples of linear interpolation include time-domain interpolation and frequency-domain interpolation. For example, the method of estimating the channel response of the sub-channel C (1) at time point t2 is shown in FIG. 2. Referring to FIG. 1. The black points denote response signals, so the channel responses of the sub-channels are known. For example, the channel response of the sub-channel C (3) is A31*exp(jθ31) at the time point t1, the channel response of the sub-channel C (3) is A35*exp(jθ35) at the time point t5, wherein A is an amplitude response of the sub-channel, and θ is a phase response of the sub-channel. Firstly, the method begins at step 201, since the ratio of the difference between the time point t2 and the time point t1 to the difference between the time point t2 and the time point t5 is 1:3, the amplitude response A32 of the sub-channel C (3) at time point t2 is expressed as A32=(A31*¾+A35*¼) and the phase response θ32 of the sub-channel C (3) at time point t2 is expressed as θ32=(θ31*¾+θ35*¼) by time-domain linear interpolation.
Next, proceed to step 203, since the ratio of the sub-carrier frequency difference between the sub-channel C (1) and the sub-channel C (0) to the sub-carrier frequency difference between the sub-channel C (1) and the sub-channel C (3) is 1:2, the amplitude response of the sub-channel C (1) at time point t2 is expressed as A12=(A02*⅔+A32*⅓) and the phase response of the sub-channel C (1) at time point t2 is expressed as θ12=(θ02*⅔+θ32*⅓) by frequency-domain linear interpolation.
However, the above channel estimation obtained by linear interpolation is not actual channel response, so the estimation is not precise enough and the quality of the received signals is affected.